Due to its excellent performance, zeolite molecular sieves have a wide range of applications in refining, petrochemical, organic synthesis and many other fields. Among zeolite molecular sieves, a ZSM-5 molecular sieve has good catalytic activity, stability and shape selectivity because of its unique pore structure. In addition, when compared with a large-crystal-grain ZSM-5 molecular sieve, a small-crystal-grain molecular sieve has an even larger external surface area, which allows more active center to be exposed on the external surface. Moreover, a small-crystal-grain molecular sieve has short intra-crystalline porous channels. As a result, reactant molecules and product molecules have a lower diffusion resistance and high diffusion rate in the porous channels. A small-crystal-grain molecular sieve has short intra-crystalline porous channels and high diffusion rate in the channels. Accordingly, the generated reaction product can quickly diffuse out from the channels, which can prevent it from having any side reaction, and accordingly it helps to improve the selectivity of the product. On the other hand, a small-crystal-grain molecular sieve can help to reduce carbon deposition in channels and reduce catalyst coking and deactivation. The small-crystal-grain ZSM-5 molecular sieve has shown relatively good catalytic performance in most catalytic reactions, in which it has greatly improved the activity and selectivity of catalysts. However, on the other hand, the synthesis of the ZSM-5 molecular sieve typically has problems including complex synthetic process and long synthetic period. As a result, the rapid synthesis of the small-crystal-grain ZSM-5 molecular sieve has important practical significance.
To date, there are many reports on the synthesis of the small-crystal-grain ZSM-5 molecular sieve. The primary approaches employed in the preparation are shown in the following aspects: 1) Add nanoscale zeolite seeds into the synthetic system to synthesize a directing agent, so as to induce the formation of the small-crystal-grain ZSM-5 molecular sieve. In CN1958453A, a silicon source is dissolved in an acidic solution, and then add a templating agent; next slowly add the aqueous solution of aluminum source in the solution of silicon source, and at the same time, add the molecular sieve seed, and then carry out a hydrothermal crystallization process at a temperature of 100 to 200° C. for 10 to 60 h, so as to obtain a ZSM-5 molecular sieve having a particle size within the range of from 100 to 400 nm. (2) Introduce an inorganic salt or surfactant, etc. into the synthetic system to regulate the synthesis of the small-crystal-grain ZSM-5 molecular sieve. CN101182004B discloses a process for preparing a synthetic nanoscale molecular sieve, in which a hydrocarbon component and a surfactant are added into a precursor solution, so as to form an oil-coated melt super-compatibilizer system to effectively control the growth of the molecular sieve. (3) Regulate and control the synthetic condition, in particular using a low temperature to facilitate nucleus formation, so as to form a small-crystal-grain ZSM-5 molecular sieve. CN101279746 proposes to employ a variable temperature crystallization method to form the ZSM-5 molecular sieve, in which a short chain amine is used as the template, and the reaction mixture is first aged at a temperature of 115 to 125° C. for 1 to 4 h, and then transferred to a temperature of 230 to 250° C. for crystallization 4 to 8 h. The sizes of the crystal grains of the ZSM-5 molecular sieves prepared through the process mentioned above are various, from dozens of nanometers to hundreds of nanometers. However, the synthetic process and preparation processes are quite complex and the time period of the process is very long, at least longer than 10 h. In addition, the products can be formed only when adding the crystal seeds and other materials. The entire preparation process is time- and energy consuming, and thus wastes lots of energy, time and at the same time, contaminates the environment.
In our prior patent entitled “One-step preparation method for empty shell-type small-grain ZSM-5 molecular sieve” (Patent no.: 201410361785.5), the empty shell-type small-grain ZSM-5 molecular sieve is obtained by way of regulating the synthesis solution and crystallization growth process. However, this method needs a long time period for synthesis, which is typically 24 h (at least longer than 6 h). In addition, the preparation process is relatively time- and energy consuming.